An elevator system generally comprises a car for transporting persons or loads, which is raised, lowered or kept at a height by way of a driving means, for example a traction cable. For this purpose a drive means applies a corresponding traction force to a driving means. The elevator system is usually designed for transporting a permissible useful load or rated load. In normal use of the elevator system the load varies between zero (empty) and the rated load.
The drive means comprises a motor, the drive output torque or lifting force of which is converted into a traction force on the driving means. This motor can in that case exert, by virtue of its construction, a defined maximum lifting force in continuous operation or operation for a time. For example, the heat dissipation limits the continuous power of electric motors in continuous operation. In operation over a time, during which the motor can for a short time usually apply a higher lifting force, the maximum power take-up limits the maximum lifting force.
The static holding force for holding the car at a height can similarly be applied by the motor or advantageously by a brake, which can be integrated in the motor or can separately apply a holding force to the driving means. Since brakes with simple means can apply high brake (holding) moments, the static holding force generated by the brake is usually greater than the (continuous) lifting force able to be applied by the motor.
For reducing the holding or lifting force to be produced by the drive means it is known from, for example, U.S. Pat. No. 5,984,052 to so couple a counterweight with the car by way of a support means that it rises when the car lowers and lowers when the car rises. The support means can be identical with the driving means or separate therefrom and fixedly connected with the car and/or the drive. For the sake of simplicity, driving means is used herein interchangeably with the term “support means”.
The weight of this counterweight is usually so selected that it substantially corresponds with the sum of the empty weight and half the rated load of the car. The maximum traction force which the drive means has to apply for raising, holding or lowering the car is thus minimized. At half rated load the elevator system is balanced, i.e. the drive means does not have to apply a holding force and only friction forces have to be overcome when raising or lowering. The maximum traction force then occurs when the car is empty (in the case of which the counterweight pulls downwardly) and a full car (in the case of which the car pulls downwardly). The drive means is in that case selected so that on the one hand it can apply this maximum traction force as a static holding force and on the other hand compensation can additionally also be provided for the inertia forces, which arise at a nominal speed profile, of the car inclusive of load as well as of the counterweight in continuous lifting operation or lifting operation for a time.
In departure therefrom U.S. Pat. No. 5,984,052 proposes selecting the counterweight so that it corresponds with the sum of the empty weight and a statistical mean value of the load distribution, which in the example of embodiment is assumed as 30% of the rated load. Such an elevator system is balanced at the statistical mean, i.e. requires only small holding and lifting forces during a large proportion of the daily operation. Insofar as, however, the car in the example of embodiment conveys more than 40% of the rated load, the traction force to be applied by the drive means increases relative to the previously described elevator system balanced at 50% and exceeds, from 80% of the rated load, the maximum traction force, which can be applied, of the elevator system balanced at 50%.
In this region the same drive means can no longer provide compensation for the same inertia forces. Accordingly, U.S. Pat. No. 5,984,052 proposes changing the nominal speed profile from a specific percentage load value and continuing to operate only with lower accelerations.
The balancing proposed by U.S. Pat. No. 5,984,052 disadvantageously requires complicated empirical determination of the load mean value. Insofar as the load distribution in actual operation departs from the distribution fundamental to the design of the weight of the counterweight, the elevator system operates in sub-optimal manner. In addition, in the case of a large standard deviation from the mean value, i.e. if loads strongly deviating from the mean value frequently occur, the efficiency of this elevator system worsens.
The conventional 50% balancing requires relatively large counterweights. These are disadvantageous in production, mounting and maintenance. In particular, large counterweights disadvantageously require additional constructional space in the elevator shaft. The balancing at a statistical mean value of load considerably reduces transport capacity in full-load operation, since the nominal speed is reduced just in this operational state.